The proposed work will focus on the physiology and genetics of Streptococcus sanguis, specifically on the arginine deiminase system, with a view to developing implantable strains to moderate the pH drop in dental plaque after sugar challenge and to accelerate the subsequent pH rise. The bases for the proposal are the past successes of others in implanting variant strains of S. sanguis in humans and animals and our isolation of variants of the bacterium nonrepressible for the arginine deiminase system. These variants are able to produce ammonia from arginine while they are producing acids from dietary sugars. The ammonia partly neutralizes the acids and moderates the pH drop in cultures of the variants. The first phases of the work will involve studies of the basic physiology of the arginine deiminase system, including determinations of the range of substrates and inducers, the final products of arginine degradation, the nature of feedback inhibition and of catabolite repression, and the limiting steps in ammonia production. Then, a genetic characterization of the system will include fine-structure mapping of the genes of the system by transformation analysis, cloning of the genes on plasmids and studies of the genetic regulatory elements for the system. The prime objective of the physiological and genetic work will be to construct nonrepressible strains with enhanced capacities to produce ammonia from arginine or from peptides such as sialin. The constructed strains will then be tested for their abilities to compete with other S. sanguis strains and with Streptococcus mutans in in-vitro plaque in continuous culture. Finally, attempts will be made to implant the constructed strains in rats prior to implantation into human plaque. The genetic work will be carried out in conjunction with the Microbial Genetics Facility of the Cariology Center and the implantation studies in conjunction with the Animal Facility of the Center.